1. Field of the Invention
The present invention relates to an offset adjusting circuit for an optical disc and an offset adjusting method.
2. Description of the Related Art
Generally, in an optical disc playback (and recording) apparatus (hereinafter referred to as an “optical disc apparatus”), after amplifying, for example, four output signals from an optical pickup, a combined RF signal is obtained by adding those signals in an adder (See, for example, Japanese Patent Application Laid-open Publication No. 10 105997 (FIG. 1)).
Specifically, as shown in FIG. 6, the optical disc apparatus irradiates an optical disc 105 with one spot of a light beam. A photo-detector group of an optical pickup 200 receives reflected light from this optical disc 105. Based on the reflected light, the optical pickup 200 supplies various output signals to an RF signal processing circuit 110. Based on these output signals, the RF signal processing circuit 110 generates an RF signal and various error signals. Based on these error signals, a servo signal processing circuit 140 carries out servo control to prevent the degradation of accuracy in focusing servo and tracking servo of an objective lens of the optical pickup 200. Note that a recording system REC, as known well, is realized by constituents indicated by reference numerals 112 through 118 of FIG. 6.
A playback system PB of this optical disc apparatus will be described. The RF signal processing circuit 110 supplies the RF signal, a combined signal, to a decoder 120. The decoder 120 performs processing such as de-interleave processing, decoding for error correction, EFM demodulation, and the like on the RF signal and supplies reproduced data to a memory 121.
The memory 121 is controlled in terms of write-in and read-out of data by a system controller (microcomputer) 150, and the reproduced data is written therein from the decoder 120. Also, the reproduced data is consecutively read out from the memory 121 at a constant bit rate. The reproduced data consecutively read out from the memory 121 is supplied to a decoder 122. When the reproduced data is compressed data, this decoder 122 decompresses the data to, for example, four times its size. Digital data from the decoder 122 is supplied to a D/A converter 123 to be converted into an analog signal, which is drawn to the outside through an output terminal 124.
Here, the RF signal processing circuit 110 outputting the RF signal will be described. As shown in FIG. 7, an output signal A obtained from the optical pickup 200 is input to a resistor R10a and an output signal B obtained from the optical pickup 200 is input to a resistor R10b. Also, an output signal C obtained from the optical pickup 200 is input to a resistor R10c and an output signal D obtained from the optical pickup is input to a resistor R10d. Note that these output signals A to D are, as known well, among the output signals obtained from the detector of the optical pickup 200 and described, for example, on page 218 of a literature “Illustrated Compact Disc Reader”, Ohmsha, Ltd., Japan, Jun. 20, 2002, the First Impression of the Third Edition.
With regard to an offset-adjustment differential operational amplifier OP1, a combined signal AB of the output signals A and B is input to the inverting input terminal thereof through the resistors R10a and R10b, and a fixed reference voltage Vref0 is applied to the non-inverting input terminal of the amplifier OP1. This offset-adjustment differential operational amplifier OP1 outputs an amplified signal A′B′ to an adding circuit SUM, which signal is expressed by an operational expression:R20ab×(1/R10a+1/R10b+1/R20ab)×{Vref−(voltage value of output signal A/R10a+voltage value of output signal B/R10b)/(1/R10a+1/R10b+1/R20ab)}.
In contrast, with regard to an offset-adjustment differential operational amplifier OP2, a combined signal CD of the output signals C and D is input to the inverting input terminal thereof through the resistors R10c and R10d, and the fixed reference voltage Vref0 is applied to the non-inverting input terminal of the amplifier OP2. This offset-adjustment differential operational amplifier OP2 outputs an amplified signal C′D′ to the adding circuit SUM, which signal is expressed by the operational expression:R20cd×(1/R10c+1/R10d+1/R20cd)×{Vref−(voltage value of output signal C/R10c+voltage value of output signal D/R10d)/(1/R10c+1/R10d+1/R20cd)}.
The adding circuit SUM adds the input amplified signals A′B′ and C′D′ to output a combined signal ABCD. This output combined signal ABCD is input to an external capacitor C0 through a first terminal T1, so that with the direct current component removed, only the alternating current component returns to the inside of the RF signal processing circuit 110 through a second terminal T2. The returned alternating current component is superimposed on the fixed reference voltage Vref0 through a resistor R40 to be input to a buffer amplifier BA0. This buffer amplifier BA0 outputs an RF signal. This RF signal is supplied via another buffer amplifier BA1 to the decoder 120 of FIG. 6, which performs playback processing thereon, and also supplied to a flaw detection circuit DET.
This flaw detection circuit DET detects a flaw on an optical disc from which signals are read out by the optical pickup 200 based on the RF signal output from the adding circuit SUM. That is, the flaw detection circuit DET has a comparator having a reference voltage Vref1 input to the non-inverting input terminal thereof, and the RF signal from the buffer amplifier BA0 is input to the inverting input terminal of this comparator. When the intensity (voltage level) of this RF signal is less than the reference voltage Vref1, it is determined that the portion that the optical pickup 200 is irradiating with laser light is a portion with a flaw, and a signal of a “H” level is output as a flaw detection signal to the servo signal processing circuit 140. The servo signal processing circuit 140 having received this flaw detection signal performs, for example, processing for preventing malfunction caused by the effect of the flaw.
In the above related art shown in FIG. 7, for the combined signal ABCD obtained from the adding circuit SUM, the direct current component thereof is removed by the external capacitor C0. Then, the signal is forcibly superimposed on the fixed DC voltage Vref0 via the resistor R40. By this means, an attempt to remove the DC offset of the combined signal of the adding circuit SUM as much as possible is made.
However, the DC offset cannot be adjusted flexibly only by superimposing the signal on the fixed DC voltage Vref0. Specifically, because there is no flexibility in DC offset adjusting function, various problems occur. That is, if variations occur in the intensity of the reflected light because of difference in the specifications or the like of the optical pickup or the type of optical disc, the output signals from the optical pickup themselves cannot be accurately adjusted in terms of their offsets. As a result, the RF signal is generated based on the output signals whose offsets have not been accurately adjusted.
Therefore, at the stage where the differential operational amplifiers generate the combined signal of the output signals A to D of the optical pickup for generating the RF signal, each of the output signals A to D deviates from the dynamic range of the processing system including the differential operational amplifiers due to the insufficient adjustment of the offsets, so that the waveform thereof may be distorted. In particular, because a dynamic range of a circuit system operating at a low power supply voltage is relatively narrow with a small margin, the possibility of the waveform distortion becomes high.
As described above, as a result of not being able to generate a precise RF signal, the degradation of accuracy in processing in the playback processing system and the flaw detection circuit is caused. In particular, in the flaw detection circuit DET, accuracy in level-setting for flaw detection cannot be improved, thus causing the degradation of accuracy in flaw detection.